1. Field of the Invention
The invention relates generally to stators for use with positive displacement drilling motors. More specifically, the invention relates to selecting an optimized liner thickness for a stator so as to increase the power available from a positive displacement motor while increasing longevity of the stator.
2. Background Art
Positive Displacement Motors (PDMs) are known in the art and are commonly used to drill wells in earth formations. PDMs operate according to a reverse mechanical application of the Moineau principle wherein pressurized fluid is forced though a series of channels formed on a rotor and a stator. The channels are generally helical in shape and may extend the entire length of the rotor and stator. The passage of the pressurized fluid generally causes the rotor to rotate within the stator. For example, a substantially continuous seal may be formed between the rotor and the stator, and the pressurized fluid may act against the rotor proximate the sealing surfaces so as to impart rotational motion on the rotor as the pressurized fluid passes through the helical channels.
Referring to FIG. 1, a typical rotor 10 includes at least one lobe 12 (wherein, for example, channels 14 are formed between lobes 12), a major diameter 8, and a minor diameter 6. The rotor 10 may be formed of metal or any other suitable material. The rotor 10 may also be coated to withstand harsh drilling environments experienced downhole. Referring to FIG. 2, a typical stator 20 comprises at least two lobes 22, a major diameter 7, and a minor diameter 5. Note that if the rotor (10 in FIG. 1) includes xe2x80x9cnxe2x80x9d lobes, the corresponding stator 20 used in combination with the rotor 10 generally includes either xe2x80x9cn+1xe2x80x9d or xe2x80x9cnxe2x88x921xe2x80x9d lobes. Referring to FIG. 3, the stator 20 generally includes a cylindrical external tube 24 and a liner 26. The liner 26 may be formed from an elastomer, plastic, or other synthetic or natural material known in the art. The liner 26 is typically injected into the cylindrical external tube 24 around a mold (not shown) that has been placed therein. The liner 26 is then cured for a selected time at a selected temperature (or temperatures) before the mold (not shown) is removed. A thickness 28 of the liner 26 is generally controlled by changing the dimensions of the mold (not shown).
A lower end of the rotor may be coupled either directly or indirectly to, for example, a drill bit. In this manner, the PDM provides a drive mechanism for a drill bit independent of any rotational motion of a drillstring generated proximate the surface of the well by, for example, rotation of a rotary table on a drilling rig. Accordingly, PDMs are especially useful in drilling directional wells where a drill bit is connected to a lower end of a bottom hole assembly (BHA). The BHA may include, for example, a PDM, a transmission assembly, a bent housing assembly, a bearing section, and the drill bit. The rotor may transmit torque to the drill bit via a drive shaft or a series of drive shafts that are operatively coupled to the rotor and to the drill bit. Therefore, when directionally drilling a wellbore, the drilling action is typically referred to as xe2x80x9cslidingxe2x80x9d because the drill string slides through the wellbore rather than rotating through the wellbore (as would be the case if the drill string were rotated using a rotary table) because rotary motion of the drill bit is produced by the PDM. However, directional drilling may also be performed by rotating the drill string and using the PDM, thereby increasing the available torque and drill bit rpm.
A rotational frequency and, for example, an amount of torque generated by the rotation of the rotor within the stator may be selected by determining a number of lobes on the rotor and stator, a major and minor diameter of the rotor and stator, and the like. An assembled view of a rotor and a stator is shown in FIG. 3. Rotation of the rotor 10 within the stator 20 causes the rotor 10 to nutate within the stator 20. Typically, a single nutation may be defined as when the rotor 10 moves one lobe width within the stator 20. The motion of the rotor 10 within the stator 20 may be defined by a circle O which defines a trajectory of a point A disposed on a rotor axis as point A moves around a stator axis B during a series of nutations. Note that an xe2x80x9ceccentricityxe2x80x9de of the assembly may be defined as a distance between the rotor axis A and the stator axis B when the rotor 10 and stator 20 are assembled to form a PDM.
Typical stators known in the art are formed in a manner similar to that shown in FIG. 2. Specifically, an inner surface 29 of the external tube 24 is generally cylindrical in shape and the stator lobes 22 are formed by molding an elastomer in the external tube 24. Problems may be encountered with the stator 20 when, for example, rotation of the rotor 10 within the stator 20 shears off portions of the stator lobes 22. This process, which may be referred to as xe2x80x9cchunking,xe2x80x9d deteriorates the seal formed between the rotor 10 and stator 20 and may cause failure of the PDM. Chunking may be increased by swelling of the liner 26 or thermal fatigue. Swelling and thermal fatigue may be caused by elevated temperatures and exposure to certain drilling fluids and formation fluids, among other factors. Moreover, flexibility of the liner 26 may lead to incomplete sealing between the rotor 10 and stator 20 such that available torque may be lost when the rotor compresses the stator lobe material, thereby reducing the power output of the PDM. Accordingly, there is a need for a stator design that provides increased power output and increased longevity in harsh downhole environments.
In one aspect, the invention comprises a stator for a positive displacement motor. The stator comprises an external tube comprising an outer surface and an inner surface, and the inner surface comprising at least two radially inwardly projecting lobes extending helically along a selected length of the external tube. A liner is disposed proximate the inner surface, and the liner conforms to the radially inwardly projecting lobes formed on the inner surface and to the helical shape of the inner surface. A thickness of the liner is at a maximum proximate the at least two radially inwardly projecting lobes.
In another aspect, the invention comprises a positive displacement motor. The positive displacement motor comprises a stator including an external tube comprising an outer surface and an inner surface. The inner surface comprises at least two radially inwardly projecting lobes extending helically along a selected length of the external tube. A liner is disposed proximate the inner surface, and the liner conforms to the radially inwardly projecting lobes formed on the inner surface and to the helical shape of the inner surface. A thickness of the liner is at a maximum proximate the at least two radially inwardly projecting lobes. A rotor is disposed inside the stator, and the rotor comprises at least one radially outwardly projecting lobe extending helically along a selected length of the rotor. The at least one radially outwardly projecting lobe formed on the rotor is adapted to sealingly engage the at least two radially outwardly projecting lobes formed on the liner.
Other aspects and advantages of the invention will be apparent from the following description and the appended claims.